What is a winding error and how do I fix it??

Hi guys, Ive been recently trying to upload a vrml model for colour printing to shapeways but I keep getting a "winding error" can someone enlighten me as to what this means and how it can be fixed?

Btw, Im using blender so any blender specific help would be appreciated but not essential

I don't use Blender, but I have had models with winding check errors which I corrected by extruding a pseudo 2D surface with AccuTrans3D

A winding error means that one or more normals are pointing in the wrong direction.

But we now have automatic fixing of color models upon upload available. You should try again. Most likely it will be successfully processed now.

- Robert

Thanks both of you Ill try again Robert and cross my fingers